The crew must discover which of them has been possessed by a parasitic Energy Rider, before the being that has come to find it kills them all. Meanwhile, Scorpius finds a deadly problem with his wormhole research.

After losing Teal'c to Apophis, SG-1 and Jacob prepare to battle against their foe on board Cronus' ship. A mysterious ship appears which allows them to escape only slightly damaged. A later encounter with the enemy ship shows no signs of life on board, save for an army of replicator bugs and an auto destruct programme.